1. Field of the Invention
The invention relates to a surface light source device and a display device. Specifically, the invention related to a surface light source device equipped with minute unevenness for prevention of reflection on the observation side surface of a transparent plate or a light emitting surface.
2. Description of the Related Art
The long-life of a battery is required in mobile instruments equipped with a display such as a liquid crystal display device. Since a reflection type liquid crystal display device can use a combination of a light source with natural light for illuminating a screen and power consumption is little, it has been marked.
Herein, a conventional example using a surface light source device as a front light is illustrated. FIG. 1 is the schematic sectional view of a reflection type liquid crystal display device 1 comprising a front light 2 and a reflection type liquid crystal display panel 3. In the front light 2, light emitted from a light source 4 repeats total reflection in a transparent plate 5 to conduct light, and then light which was reflected by a surface deflecting pattern 8 and about vertically injected on a light emitting surface 7 (rear surface) is emitted from the light emitting surface 7. The light emitted from the light emitting surface 7 of the front light 2 passes the glass substrate of the liquid crystal display panel 3 and a liquid crystal layer as shown in an arrow mark of a real line of FIG. 1, is reflected by a reflection surface 8, passes the liquid crystal layer and the like, and returns to an original direction. Thus, after the light reflected in the inside of the liquid crystal display panel 3 is modulated by the liquid crystal display panel 3, it passes the front light 2 and emitted as an image light 9 to an observer side.
On the other hand, a portion of light which is reflected by the deflecting pattern 6 in the transparent plate 5 to be oriented to the light emitting surface 7 is reflected by Fresnel reflection on the light emitting surface 7 as shown in the arrow mark of a broken line of FIG. 1, and directly emitted to an observer side.
In general, about 4% of the light injected on the light emitting surface 7 becomes noise light 10 by total reflection, and when the noise light 10 is generated, the noise light 10 and the image light 9 are emitted to the same direction as shown in FIG. 1; therefore white light is duplicated on the image prepared by the liquid crystal display panel 3, and the contrast of the screen is lowered to deteriorate visibility.
The view shown in FIG. 2 is the schematic sectional view of the reflection type liquid crystal display panel 11 by which the above-mentioned deterioration of visibility was prevented. A reflection preventive pattern 13 consisting of minute unevenness 12 which were arranged at a period P of the wavelength of light or less is provided at the light emitting surface 7 of the transparent plate 5 in the reflection type liquid crystal display panel 11. FIG. 3 is a magnified perspective view showing one portion of contours of the reflection preventive pattern 13 which was provided on the light emitting surface 7 of the transparent plate 5, and the pyramidal minute unevennesses 12 are arranged at a fixed period p (pitch). The reflection preventive patterns or a front light having the minute unevenness are known in the conventional art.
FIG. 4 is the illustration view of the action of the reflection preventive patterns 13. FIG. 4A represents the section of the transparent plate 5 which has a refractive index of n1 (>a refractive index of air n0), and the width of each of the minute unevennesses 12 of the reflection preventive patterns 13 is gradually narrowed to the lower end portion. In a zone at which the reflection preventive patterns 13 is formed, since the volume ratio of a medium (a transparent plate material) having a refractive index of n1 to a medium (air) having a refractive index of n0 is gradually varied depending on the thickness direction of the transparent plate 5, the effective refractive index of the medium is gradually varied from the refractive index n1 of the transparent plate 5 to the refractive index n0 of air in accordance with an orientation from upward to downward as shown in FIG. 4B.
Herein, when the period of the minute unevenness 12 is set as p and the wavelength of visible light having the shortest wavelength among light emitted from the light source 4 is set as λmin, it is desirable to satisfy the following condition:p<λminin order to reduce the reflected light (noise light).
When λmin is the wavelength in vacuum, it is more desirable that the condition of the period p of the minute unevenness 12 is set as follow considering that the wavelength is shortened to λmin/n1 in the transparent plate:P<λmin/n1.
However, according to the reflection type liquid crystal display panel 11, since the minute unevennesses 12 are formed at a period p of the wavelength of light or less and the (effective) refractive index of the medium to a thickness direction is continuously varied, the Fresnel reflection in the light emitting surface 7 is decreased over a wide wavelength region and the contrast of an image is improved when light is vertically injected from upside to the reflection preventive pattern 13 as shown in the real line arrow mark of FIG. 3, and when light is injected to the reflection preventive pattern 13 at a smaller incident angle than the critical angle of total reflection. The behavior of light in the reflection type liquid crystal display panel 11 at this time is shown in FIG. 2.
Thus, the method has been known that the contrast of an image is improved by providing the reflection preventive pattern 13 on the light emitting surface 7 to suppress the Fresnel reflection.
However, when the present inventors have tried to study a further better image of the liquid crystal display device, they have found that the diffracted light which is generated at the reflection preventive pattern 13 is one of main causes deteriorating the contrast of a screen. Namely, when the reflection preventive pattern 13 is provided on the light emitting surface 7 of the transparent plate 5, the Fresnel reflection can be suppressed but the reflection preventive patterns 13 in which the minute unevennesses 12 are arranged at a constant period work as diffraction grating; therefore when light in the transparent plate 5 is emitted from the light emitting surface 7, diffracted light is generated. Thus, the diffracted light emitted from the light emitting surface 7 of the transparent plate 5 is emitted for an observer side directly or in irregular reflection. The diffracted light is duplicated with the image light 9 of a screen and deteriorates the contrast of an image, further the transparent plate is subject to a tone of color, and it deteriorates the visibility of the liquid crystal display panel 11.
Further, according to the study by the present inventors, it has been grasped that the generation of the diffracted light on the reflection preventive patterns 13 is generated by the function peculiar to the surface light source as shown below. Namely, it is sufficient to consider incident light from an about vertical direction for the reflection preventive pattern (minute unevenness) for general use as shown in the real line arrow mark of FIG. 3. To the contrary, as shown in the broken line arrow mark of FIG. 3, it is required to consider light injected at a large incident angle on the light emitting surface 7 for the reflection preventive pattern 13 which was provided on the rear surface of the transparent plate 5. As shown in FIG. 5, the surface light source device 2 has a function that the total reflection of light emitted from the light source 24 is carried out on the surface and the rear surface of the transparent plate 5, and the light is transmitted and uniformly emitted from the whole surface. Accordingly, light with large intensity is injected for the reflection preventive pattern 13 at a larger incident angle than the critical angle of the total reflection. On the other hand, when the reflection preventive pattern 13 is viewed from a direction vertical to the light emitting surface 7, the effective refractive index is the maximum at the center of the minute unevenness 12 because the effective refractive index of the respective minute unevennesses 12 becomes large at a portion with a large thickness. The effective refractive index becomes small at a surrounding portion of the minute unevenness 12, and the distribution of the effective refractive index as shown in FIG. 6B is shown. Consequently, when the reflection preventive pattern 13 is viewed from a vertical direction, it can be estimated that the reflection preventive pattern 13 is a two dimensional diffraction grating as shown in FIG. 6B. Accordingly, when light which is in nearly parallel with the reflection preventive pattern 13 is injected, it is diffracted by the minute unevenness 12 which was arranged two dimensionally, and as shown in FIG. 5, the diffracted light 14 is emitted from the light emitting surface 7. Since the diffracted light 14 is emitted to an observer side directly or in irregular reflection, the diffracted light is duplicated with the image light of a screen and the transparent plate wears a tone of color, and the contrast of an image is lowered to deteriorate the visibility of the liquid crystal display panel 11.
Further, a method of forming a dielectric multilayer on the light emitting surface can be considered as a method of preventing the Fresnel reflection of light on the light emitting surface of the transparent plate, but there are problems that the method is complicated in a film forming process and costs high, and environmental resistance is inferior.